Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus having a function of detecting a potential of an image bearing member by detecting a current flowing to the image bearing member through a member.
Description of the Related Art
A contrast of an image formed by an image forming apparatus is determined based on a potential difference between a developing voltage and a surface potential of a photosensitive drum (hereinafter referred to as “photosensitive drum potential”) exhibited after laser light is emitted thereto from a laser light source. However, the contrast of the image varies depending on an environment (for example, temperature and humidity) and a film thickness of the photosensitive drum, which necessitates a correction of the contrast. Therefore, in order to detect an actual photosensitive drum potential and enable a correction with high accuracy, an image forming apparatus configured to detect the photosensitive drum potential as follows is proposed in, for example, Japanese Patent Application Laid-Open No. 2012-13881. That is, an AC voltage is applied to the photosensitive drum from an application circuit for a charging voltage through a charge roller, and a residual potential on the photosensitive drum is eliminated. After that, DC voltages having a positive polarity and a negative polarity are applied to the photosensitive drum from the application circuit for the charging voltage through the charge roller, and discharge start voltages having a positive polarity and a negative polarity of the photosensitive drum are measured. The surface potential of the photosensitive drum is detected based on the measured discharge start voltages.
In a related-art configuration, the charging of the photosensitive drum and the detection of the photosensitive drum potential exhibited after the laser irradiation are conducted through the charge roller. Therefore, the photosensitive drum potential cannot be detected until a surface position in which the photosensitive drum has been charged by the charge roller returns to the position of the charge roller again after the photosensitive drum is rotated by one revolution, which requires time for the detection of the photosensitive drum potential. Further, for improvement in detection time, there is also a system that uses a transfer roller serving as a transfer member to detect the photosensitive drum potential exhibited after the laser irradiation, but a detection result of the photosensitive drum potential needs to be corrected. In order to calculate an amount (correction amount) by which the detection result of the photosensitive drum potential is to be corrected, an AC voltage needs to be applied by the charge roller. Therefore, a circuit configured to generate an AC voltage needs to be provided, which increases cost of a circuit configuration.